Welcome,

By your description, and without pictures my guess is also that the auxiliary transmission is Spicer 5831. The only info that will help figure out what ratios will be stamped on the small aluminum tag on one side near the top. Click my picture link below and look in the "auxiliary transmission" folder, I have pictures of a 5831, and others. If you can find the tag I can probably tell you what ratios.

The entire drivetrain should be fine behind a V8.

The vacuum lever under the dash is for high and low in the rear axle, a two speed rear.
There is also a vacuum line that goes to the Hydrovac, one of the parts of the system on the firewall is a check valve.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-